There is a service which uses FM broadcast radio waves as media to transmit service information, such as character information, multiplexed on an FM broadcast radio wave and display the service information on display means provided on a FM radio receiver. This service is called "FM teletext broadcasting" which has already been put to a practical use as visual information radio.
According to this FM teletext broadcasting, an FM teletext broadcast radio wave is acquired by frequency-multiplexing display information such as characters and/or numerals on an ordinary FM broadcast radio wave. The acquired FM telecast broadcast radio wave is transmitted from an FM broadcasting station. The FM teletext broadcast radio wave is received by an FM radio receiver with a display like a liquid crystal display and is separated to audio information and display information. The display information is converted to character codes to be displayed on the display to allow a user to see this display information. Accordingly, the user can see information associated with broadcasting, such as the title of a broadcast music piece, the name of the artist, comments and/or the number of requests, and news, weather information, traffic information or the like, which is not directly associated with broadcasting, as character information.
In FM teletext broadcasting, there is specific service information which may be charged as well as charge-free service information. Such charged service information is scrambled on the FM broadcasting station and is transmitted. While a receiver of a subscriber for charged broadcast programs is provided with means for storing or inputting a descramble key to descramble (decode) scrambled information so that charged service information can be descrambled for visual purpose.
If anyone who is not subscribed for charged broadcasting decodes the descramble key to descramble charged service information, however, unauthorized watching of the charged service information becomes possible.